A robust sequence synchronization unit for multi-user DS-CDMA chaos-based communication systems

This paper demonstrates a way of achieving and maintaining sequence synchronization in multi-user direct sequence code division multiple access (DS-CDMA) based chaotic communication systems. Synchronization is achieved and maintained through the code acquisition and the code tracking phase, respectively. The performance of the proposed system is evaluated in the presence of additive white Gaussian noise and interuser interferences. Throughout, a pseudo-random binary sequence (PRBS) is used as the synchronizing pilot signal within the multi-user chaotic communication system. In addition, the Logistic and Bernoulli chaotic maps are also used as the pilot signals in the investigation of the code acquisition performance. The code acquisition circuit is evaluated in terms of the probability of detection and probability of false alarm. The corresponding results demonstrate an ability to achieve initial synchronization. Furthermore, it is shown that in terms of code acquisition, the PRBS outperforms the Logistic and Bernoulli chaotic maps. A mathematical model of the code tracking loop is then presented. From the model, a control law for the generation of time offset estimates is derived. The robustness of the synchronization unit is then demonstrated in terms of the bit error rate. It has been shown that for the case of 1, 2, 3, 4, and 5 users, the bit error rate goes below the maximum acceptable limit of 10−3 at the bit energy to noise power spectral density ratio of approximately 8, 9, 9.5, 11, and 12 dB, respectively. Furthermore, a gradual degradation in performance, above the maximum acceptable bit error rate limit, is demonstrated for the increasing number of users.